1. Field of the Invention
The present invention relates to a switch for use in a high magnetic field which is generated from a high magnetic field generator, such as a magnetic resonance imaging apparatus.
2. Description of the Related Art
A known magnetic resonance imaging apparatus uses a high-intensity static magnetic field generator including either one or a combination of a super-conducting magnet, resistive magnet and permanent magnet. The high-intensity static magnetic field generator is adapted to generate thousands to tens of thousands of magnetic field within a spherical space some tens in diameter in a stable fashion over a length of time. The direction of such high-intensity static magnetic field is substantially constant within the space.
In a medical magnetic resonance imaging apparatus using this type of high-intensity static magnetic field apparatus, a switch is sometimes located within the high-intensity static magnetic field apparatus, as will be set forth below by way of example. The general medical magnetic resonance imaging apparatus comprises a high-intensity static magnetic field generator, transmitting and receiving system, field gradient generation system, control system and signal processing system. The transmitting and receiving system comprises, as main elements, transmitting coil, receiving coil, a transmit/receive coil (i.e., a single coil) and a plurality of high frequency transmission paths and is located substantially within the high-intensity static magnetic field generator. One or more switches are assembled for some main elements and an assembly thus obtained is associated with, for example, a plurality of capacitors in the aforementioned coil to allow a switching to be made among a corresponding number of the high frequency transmission paths. In this way it is possible to improve the transmission/reception characteristics and select a desirable atomic nucleus, an object to be imaged.
The characteristics of the aforementioned switch are of such a type that it is possible to reliably open or close an associated circuit located as close to the coil as possible and to maintain that state. In this case, the coil needs to be located below the high magnetic field atmosphere, that is substantially within the high-intensity static magnetic field generator.
As a general candidate switch use may be made of, for example, an electromagnetic relay, that is a switch capable of opening and closing the associated contact by remote control. However, the aforementioned electromagnetic relay suffers various restrictions regarding its installation.
For example, the first restriction is that the operation of the switch is affected by the high-intensity static magnetic field which is generated from the high-intensity static magnetic field generator. The second restriction is that uniformity is not disturbed in the high-intensity static magnetic field due to the use of the switch. The third restriction is that a subject (sample) can be positioned in not too narrow a space in spite of providing the switch in the high-intensity static magnetic field generator.
The third restriction presents no problem because an inexpensive, adequately compact, easy-to-handle electromagnetic relay can readily be obtained irrespective of the size of a current capacity. The second restriction, on the other hand, may disturb the high-intensity static magnetic field due to the presence of an associated electromagnetic actuator, support member, etc., which are made of ferromagnetic material and incorporated into the electromagnetic relay. The first restriction presents a problem because the aforementioned electromagnetic actuator, support member, etc., may induce an operation error, or not operate at all, due to an action of an electromagnetic force resulting from the high-intensity static magnetic field applied.
With this in view, it may be difficult to mount, in place, and use this type of electromagnetic relay under the aforementioned high-intensity static magnetic field atmosphere. As a consequence, this type of electromagnetic relay has to be placed in a specific position below a low magnetic field atmosphere of several gausses, that is, in a position greatly remote from the coil. It is, therefore, impossible to either obtain the desirable receive characteristics due to a transmission loss responsible for an extension of a cable from the receiving coil to the preamplifier, or to select the type of the atomic nucleus, an object to be imaged, under a preferable condition.